1. Field of the Invention
This invention relates to a high tension cable device and a process of producing the same, and more particularly to a high tension cable device suitable for use with an ignition system with a series gap for an automobile engine or the like and a process of producing the same.
2. Description of the Prior Art
Ignition systems for an automobile engine or the like are constituted such that a high voltage is applied to an ignition plug to produce a spark. As one of such ignition systems, an ignition system with a so-called series gap has been proposed wherein a discharge gap is provided in series to an ignition plug in order to prevent occurrence of smoke upon ignition and control the ignition timing accurately.
Such ignition system with a series gap is disclosed, for example, in Japanese Utility Model Laid-Open No. 63-101486 and is shown in FIG. 7. Referring to FIG. 7, the ignition system shown includes an ignition plug A screwed, for example, in a cylinder head B of an engine, and a high tension cable device C removably fitted on the ignition plug A. The high tension cable device C includes a high tension cable 3 and a plug cap 6 connected to an end of the high tension cable 3. The plug cap 6 includes a connecting terminal 2 for the connection to a terminal 1 of the ignition plug A, and a cable terminal 4 coupled to the end of the high tension cable 3. The connecting terminal 2 and cable terminal 4 are disposed, for example, in a substantially tubular casing 5 in an opposing spaced relationship from each other in a longitudinal direction of the casing 5. Though not particularly shown, the casing 5 is composed of a first casing in which the connecting terminal 2 is mounted and a second casing in which the cable terminal 4 is mounted.
A discharge tube 10 is mounted between the connecting terminal 2 and the cable terminal 4 in the plug cap 6 such that an outer periphery thereof closely contacts with an inner periphery of the casing 5. The discharge tube 10 includes a pair of discharge electrodes 7 and 8 sealed in a glass tube 9 in such a manner as to form a series gap S therebetween. The discharge electrodes 7 and 8 of the discharge tube 10 are fitted in and electrically connected to the connecting terminal 2 and cable terminal 4 of the casing 5, respectively.
A discharge tube for use with such an ignition system with a series gap is required to have a somewhat higher starting voltage than that of an ignition plug in order to control the ignition timing accurately. However, where a discharge tube having a high starting voltage is used, a so-called creeping discharge likely occurs along an outer face of the discharge tube prior to starting of a discharge at the series gap. Accordingly, it is very difficult to miniaturize a discharge tube itself while a high starting voltage is assured, and also it is difficult to miniaturize a plug cap of a high tension cable device.
Meanwhile, since the high tension cable device C described above is mounted on an engine, various vibrations of the engine and so forth are transmitted directly to the high tension cable device C. Where the glass tube 9 of the discharge tube 10 is fitted directly in the casing 5 of the plug cap 6 as described above, such external vibrations are transmitted directly to the discharge tube 10 and may possibly cause damage to the discharge tube 10. Further, since the discharge electrodes 7 and 8 of the discharge tube 10 and the terminals 2 and 4 in the casing 5 are respectively communicated with each other through mere fitting contact between them, there is the possibility that an instantaneous disconnection arising from incomplete contact between those members may be caused by such transmission of external vibrations to the discharge tube 10 as described above.
Another high tension cable device for use with an ignition system is also known wherein a discharge tube is mounted in a plug cap such that an outer periphery thereof is spaced from an inner periphery of a casing of the plug cap. A high tension cable device of the type just mentioned is shown in FIG. 8. Referring to FIG. 8, the high tension cable device also generally denoted at C is substantially similar in construction to the high tension cable device C shown in FIG. 7 except that the discharge tube 10 is fitted in the plug cap 6 with an air gap 11 left between an outer periphery of the glass tube 9 of the discharge tube 10 and an inner periphery of the casing 5. In the high tension cable device C, the discharge tube 10 is supported only at the discharge electrodes 7 and 8 thereof which have electrode terminals 7a and 8a fitted in and connected to the terminals 2 and 4 fitted in the casing 5.
While the air gap 11 is shown in a somewhat exaggerated manner in FIG. 8, a similar air gap is inevitably formed more or less also between the outer periphery of the glass tube 9 of the discharge tube 10 and the inner periphery of the casing 5 of the high tension cable device C shown in FIG. 7. Where such air gap 11 exists, a creeping discharge likely occurs along the outer face of the discharge tube 10 prior to starting of discharging between the electrodes 7 and 8. Then, once such creeping discharge occurs, a required starting voltage cannot be obtained any more, and there is a problem that the ignition timing and so forth cannot be controlled precisely.